Gas turbine engine heat management system

ABSTRACT

A heat management system of a gas turbine engine for cooling oil and heating fuel, includes an oil circuit having parallel connected first and second branches. The first branch includes a fuel/oil heat exchanger and a first fixed restrictor in series and the second branch includes an air cooled oil cooler and a second fixed restrictor. The first and second fixed restrictors limit respective oil flows through the first and second branch differently, in response to viscosity changes of the oil caused by temperature changes of the oil during engine operation to modify oil distribution between the first and second branches.

TECHNICAL FIELD

The application relates generally to gas turbine engines, and moreparticularly, to a heat management system of a gas turbine engine forcooling oil and heating fuel.

BACKGROUND OF THE ART

A heat management system of a gas turbine engine conventionally includesa fuel/oil heat exchanger (FOHE) to transfer heat from the hot oil tothe cold fuel in order to heat the cold fuel to a desired temperature.An air cooled oil cooler (ACOC) is also conventionally provided in theheat management system to further cool the hot oil to a lowertemperature in order to be recycled in an oil circuit of the engine.ACOCs and FOHEs are conventionally connected in series and a thermalstatic valve is also provided to allow an oil flow to selectively bypassthe ACOC, for example in cold oil conditions. However, the conventionalthermal static valves generally have very low reliability, which drivesup maintenance costs.

Accordingly, there is a need to provide an improved system for gasturbine engines.

SUMMARY

In one aspect, there is provided a heat management system of a gasturbine engine for cooling oil and heating fuel, the system comprisingan oil circuit having connected first and second branches in a parallelconfiguration, the first branch including a fuel/oil heat exchanger fortransferring heat from an oil flow through the first branch, to a fuelflow and a first fixed restrictor for restricting the oil flow throughthe first branch, the second branch including an air cooled oil coolerfor air cooling an oil flow through the second branch and a second fixedrestrictor for restricting the oil flow through the second branch, thefirst and second fixed restrictors having respective fixed passagegeometries, the passage geometry of the second fixed restrictor having atotal flow contact area greater than a total flow contact area of thefirst fixed restrictor such that in response to an oil viscosityincrease, the second fixed restrictor provides a larger flow resistanceincrease than a flow resistance increase provided by the first fixedrestrictor.

In another aspect, there is provided a heat management system of a gasturbine engine for cooling oil and heating fuel, the system comprisingan oil circuit having connected first and second branches in a parallelconfiguration, the first branch including a fuel/oil heat exchanger fortransferring heat from an oil flow through the first branch, to a fuelflow and a first fixed restrictor disposed downstream of the fuel/oilheat exchanger for restricting the oil flow through the first branch,the second branch including an air cooled oil cooler for air cooling anoil flow through the second branch and a second fixed restrictordisposed downstream of the air cooled oil cooler for restricting the oilflow through the second branch, wherein the first fixed restrictorincludes a diaphragm having a flow orifice and the second fixedrestrictor includes a plurality of holes extending through a body, theholes being small and long with respect to the flow orifice of the firstfixed restrictor.

In a further aspect, there is provided a method of managing oil coolingand fuel heating in a gas turbine engine, the method comprising a)distributing oil from a pumped oil supply into first and second oilflows, the second oil flow being parallel to the first oil flow; b)transferring heat from the first oil flow to a fuel flow; c) usingambient air to cool the second oil flow; and d) using a combination oftwo fixed restrictors to limit the respective first and second oil flowsdifferently, in response to viscosity changes of the oil caused bytemperature changes of the oil during engine operation.

Further details of these and other aspects of the described subjectmatter will be apparent from the detailed description and drawingsincluded below.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying drawings depicting aspects ofthe described subject matter, in which:

FIG. 1 is a schematic cross-sectional view of an aircraft turbofan gasturbine engine as an exemplary application of the described subjectmatter;

FIG. 2 is a schematic illustration of a heat management system accordingto one embodiment;

FIG. 3 is a partial cross-sectional view of a fuel/oil heat exchanger(FOHE) restrictor showing a diaphragm having a flow orifice according toone embodiment;

FIG. 4 is a partial cross-sectional view of the FOHE restrictor showinga diaphragm having a sharp edged flow orifice according to anotherembodiment; and

FIG. 5 is a cross-sectional view of an air cooled oil cooler (ACOC)restrictor and a surface of a body with a plurality of small holesextending through the body.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION

Referring to FIG. 1, an aircraft turbofan gas turbine engine includes ahousing or nacelle 10, a core casing 13, a low pressure spool assembly(not numbered) which includes a fan assembly 14, a low pressurecompressor assembly 16 and a low pressure turbine assembly 18 connectedby a shaft 12, and a high pressure spool assembly (not numbered) whichincludes a high pressure compressor assembly 22 and a high pressureturbine assembly 24 connected by a turbine shaft 20. The core casing 13surrounds the low and high pressure spool assemblies to define a mainflow path or gas path (not numbered) therethrough. In the main flow paththere is provided a combustion gas generator assembly 26 to generatecombustion gases for powering the high and low pressure turbineassemblies 24, 18. There is also provided a fuel supply system 28 forsupplying fuel to the combustion gas generator assembly 26. There isfurther provided a heat management system 30 for cooling hot oilcirculated in an oil system (not shown) of the engine and for heatingthe fuel prior to being delivered for combustion. The heat managementsystem 30 schematically illustrated in FIG. 1, does not represent aspecific structure and location in the engine.

Referring to FIGS. 1 and 2, the heat management system 30 includes anoil circuit illustrated as a block defined by broken lines 32 in FIG. 2.In one embodiment, the oil circuit 32 may include a first branch 34 anda second branch 36 connected in a parallel configuration. The firstbranch 34 may include a fuel/oil heat exchanger (FOHE) 38 and an FOHErestrictor 40 in series. For example the FOHE restrictor 40 may bedisposed downstream of the FOHE 38. Optionally, a pressure relief valvesuch as a check-valve 42 may also be provided in the first branch 34,for example disposed downstream of the FOHE 38 and parallel to the FOHErestrictor 40.

In one embodiment, the second branch 36 may include an air cooled oilcooler (ACOC) 44 and an ACOC restrictor 46 in series. For example theACOC restrictor 46 may be disposed downstream of the ACOC 44.

The heat management system 30 may further include an oil pump 48 and oilfilter 50 which are disposed upstream of and connected to the oilcircuit 32 such that oil pump 48 pumps oil from an oil tank 52 whichcontains relatively hot oil collected from, for example bearing chambers(not shown) of the engine during engine operation, to the oil circuit32, splitting the oil into first and second oil flows passing throughthe respective parallel first and second branches 34, 36. The first andsecond oil flows from the first and second branches 34, 36 are combinedand directed into an engine oil manifold 54 which is disposed downstreamof and connected to the oil circuit 32. The engine oil manifold 54distributes the oil to various locations of the engine to lubricate andcool for example bearings and gears of the engine.

The FOHE 38 includes oil passages (not numbered) forming part of thefirst branch 34 of the oil circuit and fuel passages (not shown) whichare connected in the fuel system. Therefore, cold fuel from a fuelsupply 56 can be directed through the FOHE 38 and can be thus heated bythe first flow of the hot oil passing through the first branch 34 of theoil circuit 32. The heated fuel from the FOHE 38 may be directed forexample, through a filter 58 to an engine fuel control unit FCU 60 whichcontrols fuel delivery at a required rate to the combustion gasgenerator assembly 26.

The ACOC 44 includes oil passages (not shown) exposed to for example, anambient air stream 62 passing through a bypass duct (not numbered) ofthe engine. Therefore, the second oil flow passing through the secondbranch 36 of the oil circuit 32 is cooled by the relatively cool ambientair stream 62. The oil in the engine oil manifold 54 is a mixture of thefirst oil flow which passes through the first branch 34 of the oilcircuit 32 and is cooled in the FOHE 38 by cold fuel, and the second oilflow which passes through the second branch 36 of the oil circuit 32 andis cooled in the ACOC 44 by the cold ambient air stream 62. Therefore,the oil in the engine oil manifold 54 is cooler than the oil in the oiltank 52.

During engine operation the fuel flow required for combustion and thetemperatures of the hot oil flowing through the oil circuit may vary andtherefore the heat exchange performed in the FOHE 38 and ACOC 44 must becontrolled accordingly. A thermal static valve (also known as a thermalvalve) is conventionally used in an engine heat management system forthis purpose, as discussed above in the Background of the Art. In theheat management system 30, the thermal valve may be eliminated. The oilflow split between the FOHE 38 and the ACOC 44 is controlled by acombination of the FOHE restrictor 40 and the ACOC restrictor 46, whichlimits the first and second oil flows through the first and secondbranches 34, 36 differently, in response to viscosity changes of the oilcaused by temperature changes of the oil during engine operation.

According to one embodiment, both the FOHE restrictor 40 and ACOCrestrictor 46 are fixed restrictors which, however have different fixedpassage geometries. The passage geometry of the ACOC restrictor 46 has atotal flow contact area greater than a total flow contact area of theFOHE restrictor 40 such that in response to an oil viscosity increase,the ACOC restrictor 46 provides a larger flow resistance increase than aflow resistance increase provided by the FOHE restrictor 40, in order tochange oil flow distribution between the first and second branches 34,36 of the oil circuit 32.

In one embodiment illustrated in FIGS. 2 and 3, the FOHE restrictor 40may be a calibrated diaphragm restrictor which, for example, includes aflow chamber 64 and a diaphragm 66 disposed within the flow chamber 64as a partition. The diaphragm 66 defines an orifice 68 extendingtherethrough to allow the first oil flow in the first branch 34 of theoil circuit 32 to pass through the chamber 64. The orifice 68 iscalibrated to limit the first oil flow passing through the FOHE 38 inorder to prevent the fuel flow from being overheated. The diameter ofthe chamber 64 is much larger than the diameter of the orifice 68.Alternatively, the diaphragm 66 according to one embodiment, may definea flow orifice 70 having a sharp annular edge 72 with an edge tip angleA smaller than 90 degrees, as illustrated in FIG. 4.

The ACOC restrictor 46 on the other hand, according to one embodimentillustrated in FIGS. 2 and 5, may define a plurality of small and longholes 74 with respect to the flow orifice 68 of the FOHE restrictor 40.The small and long holes 74 extend through a body 76 which is disposedin a flow chamber 78 of the ACOC restrictor 46. Each of the small andlong holes 74 has a diameter smaller than an axial length of the hole74.

The performance of the ACOC 44 may be chosen to provide adequate oilcooling when the engine is operated at high altitudes and theperformance of the FOHE 38 is chosen to provide adequate heat transferfrom oil into fuel in cold conditions. The ACOC restrictor 46 may becalibrated to allow the second oil flow in the second branch 36 of theoil circuit 32 to flow almost unrestrictedly through the ACOC restrictor46 when the oil is very hot and thus the oil viscosity is very low. TheFOHE restrictor 40 may be calibrated in order to limit the first oilflow through the first branch 34 of the oil circuit to a rate thatavoids overheating the fuel flow when the engine is operated at highaltitudes and in order to send the rest of the oil to the second oilflow passing through the second branch 36 of the oil circuit. For agiven oil temperature, the oil flow split between the first and secondbranches 34, 36 may remain at approximately the same at any altitude. Atlower altitudes the performance of the ACOC 44 improves significantlydue to the increased air density and, consequently, increased air massflowing through the ACOC 44. The increased performance of the ACOC 44may match the increased engine heat rejection at lower altitudes, whichengine heat rejection is also proportional to the increased pressure inthe gas path of the engine. There may also be a marginal increase in theheat transferred from oil into fuel in the FOHE 38 due to the increasedfuel flow at the lower altitudes.

At a lower ambient temperature the second oil flow in the second branch36 exiting from the ACOC 44 is cooler. The ACOC restrictor 46 thereforeoffers increased flow resistance to the second oil flow in the secondbranch 36 due to increased oil viscosity when the second oil flow in thesecond branch 36 is cooler. This results in a reduction of the secondoil flow through the second branch 36 and a corresponding increase ofthe first oil flow in the first branch 34 of the oil circuit. Meanwhile,the flow resistance provided by the FOHE restrictor 40 is substantiallyindependent from the temperatures of the first oil flow flowing throughthe first branch 34 because the flow resistance determined by the fixedgeometry of the orifice 68 or 70 of the FOHE restrictor 40 is notsignificantly affected by oil viscosity changes with respect to the ACOCrestrictor 46. The second oil flow reduction in the second branch 36 ofthe oil circuit when the ambient air temperatures are low, determinesfurther oil cooling in the ACOC 44 (because not only the ambient airstream is cooler but also less oil is concurrently being cooled by thecooler air stream). At lower ambient temperatures, the temperature ofthe second oil flow exiting from the ACOC 44 reaches the ambient airtemperature and the second oil flow in the second branch 36 may bereduced to minimum while the first oil flow in the first branch 34 inthe oil circuit 32 is increased to maximum. The increased first oil flowat low ambient air temperatures may ensure that an optimum amount ofheat is transferred from the engine oil system to the engine fuelsystem.

The check-valve 42 (for pressure relief) in the first branch 34 isnormally closed and opens only when the oil pressure build-up in thefirst and second branches 34, 36 in the oil circuit 32, reaches apredetermined level, in order to prevent the FOHE 38 and ACOC 44 frombeing damaged.

Alternatively, the FOHE restrictor 40 and the check-valve 42 may becombined in one unit, such as a pressure relief valve with calibratedflow leakage.

The heat management system 30 may eliminate or reduce the requirementfor thermal valves, or for commanded/actuated control valves. Thecombination of the fixed restrictors 40, 46, with an optional pressurerelief valve, is simpler, cheaper and may have significantly betterreliability than thermal valves or commanded/actuated control valves,for controlling oil flow distribution between the ACOC and the FOHE 38,44. The heat management system 30 allows a relatively simple systemarchitecture and optimum component sizing. It should also be noted thatsince oil viscosity changes exponentially with respect to oiltemperature, the thermal control offered by the heat management system30 may therefore be quite accurate and without hysteresis.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departure from the scope of the described subjectmatter. For example, the fixed passage geometries of the respective FOHErestrictor 40 and ACOC restrictor 46 may be any suitable and may thusvary from the structures in the described embodiments. The oil circuit32 of the heat management system as described above, may bealternatively positioned to receive used hot oil from bearing chambersand to discharge cooled oil to an oil tank of the engine. Still othermodifications which fall within the scope of the described subjectmatter will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

The invention claimed is:
 1. A heat management system of a gas turbineengine for cooling oil and heating fuel, the heat management systemcomprising an oil circuit having a first branch and a second branchconnected in parallel, the first branch including a fuel/oil heatexchanger for transferring heat from an oil flow through the firstbranch to a fuel flow, and a first fixed restrictor for restricting theoil flow through the first branch, wherein the first fixed restrictorincludes a diaphragm having a flow orifice, the second branch includingan air cooled oil cooler for air cooling an oil flow through the secondbranch and a second fixed restrictor for restricting the oil flowthrough the second branch, wherein the second fixed restrictor includesa plurality of holes extending through a body, the plurality of holesbeing smaller in diameter and longer in axial length than the floworifice of the first fixed restrictor such that the second fixedrestrictor has a total flow contact area greater than a total flowcontact area of the first fixed restrictor so that in response to an oilviscosity increase, a flow resistance increase of the second fixedrestrictor is greater than a flow resistance increase of the first fixedrestrictor to modify oil flow distribution between the first and secondbranches.
 2. The heat management system as defined in claim 1 whereinthe first fixed restrictor is disposed downstream of the fuel/oil heatexchanger and the second fixed restrictor is disposed downstream of theair cooled oil cooler.
 3. The heat management system as defined in claim2 wherein the first branch further comprises a pressure relief valvedisposed downstream of the fuel/oil heat exchanger and in parallelconnection with the first fixed restrictor.
 4. The heat managementsystem as defined in claim 1 comprising an oil pump disposed upstream ofthe oil circuit.
 5. The heat management system as defined in claim 1comprising an engine oil manifold disposed downstream of the oilcircuit.
 6. The heat management system as defined in claim 1 wherein thefuel flow is directed from an engine fuel supply to an engine fuelcontrol unit.
 7. The heat management system as defined in claim 1wherein the first fixed restrictor is a calibrated diaphragm restrictor.8. A heat management system of a gas turbine engine for cooling oil andheating fuel, the heat management system comprising an oil circuithaving connected a first branch and a second branch in a parallelconfiguration, the first branch including a fuel/oil heat exchanger fortransferring heat from an oil flow through the first branch to a fuelflow, and a first fixed restrictor disposed downstream of the fuel/oilheat exchanger for restricting the oil flow through the first branch,the second branch including an air cooled oil cooler for air cooling anoil flow through the second branch and a second fixed restrictordisposed downstream of the air cooled oil cooler for restricting the oilflow through the second branch, wherein the first fixed restrictorincludes a diaphragm having a flow orifice and the second fixedrestrictor includes a plurality of holes extending through a body, theplurality of holes being smaller in diameter and longer in axial lengththan the flow orifice of the first fixed restrictor, such that thesecond fixed restrictor has a total flow contact area greater than atotal flow contact area of the first fixed restrictor so that a flowresistance increase of the second fixed restrictor is greater than aflow resistance increase of the first fixed restrictor in response to anoil viscosity increase.
 9. The heat management system as defined inclaim 8 wherein the first branch comprises a pressure relief valvedisposed downstream of the fuel/oil heat exchanger and in parallelconnection with the first fixed restrictor.
 10. The heat managementsystem as defined in claim 8 wherein the flow orifice of the first fixedrestrictor has an annular edge having an edge tip angle of less than 90degrees.
 11. The heat management system as defined in claim 8 whereineach of the holes of the second fixed restrictor has a diameter smallerthan an axial length of the hole.